1. Field of the Invention
The present invention relates to protection of metals against corrosion, and particularly to compositions intended for diffusion coating ferrous metals. Most advantageously the invention can be used for corrosion protection of parts and assemblies of the equipment used in chemical industries for the production of soda and soda products as well as magnesium chloride, barium chloride, sodium sulphates and other products of this kind.
2. Description of the Prior Art
One of the main problems facing manufacturers of chemical apparatus consists is providing of high corrosion and erosion resistance in apparatus parts contacting highly concentrated salt solutions. Experience proves that this problem has not been solved up till now.
It is well known that for the production of chemical apparatus parts having corrosion resistant coatings, cast iron and carbonaceous steels are more preferable from the economical point of view than high-cost titanium. The best results are obtained when employing chromium, titanium and their compounds in the compositions intended for protective coatings. Investigations have shown that high density of the protective layer and its strong bond with the substrate are provided by a diffusion saturation of the surface of apparatus parts with these elements and their compounds.
Known in the art as a composition for diffusion coating ferrous metals as described in U.S.S.R. Inventor's Certificate No. 443,941. The above composition comprises a particulate mixture of titanium dioxide, chromium oxide, aluminum, aluminum fluoride and alumina taken in the following ratio (in % by weight):
______________________________________ titanium dioxide 10 to 15 chromium oxide 23 to 26 aluminum 9 to 27 aluminum fluoride 3 to 5 alumina the balance ______________________________________
To form a protective coating upon workpieces, the latter are embedded into the above composition, heated to a temperature of 950.degree. to 1000.degree. C. and held at this temperature until a diffusion layer is formed. Such a coating effectively protects the substrate (of workpieces) against oxidation, gas corrosion and the influence of low-corrosive liquid media such as sea water. However in chloride containing media and in highly concentrated salt solutions the resistance of the coating is not satisfactory, which is due to the fact that aluminum can be easily corroded from the coating. In this case, selective corrosion takes place which deteriorates the continuity of the coating. In addition, the activity of the chemically bound titanium and chromium is not high which is due to a low content of these elements on the surface of the diffusion layer.
There is also known a composition for diffusion coating ferrous metals, providing for higher corrosion resistance described in U.S. Pat. No. 2,988,515. This composition comprises a particulate mixture of titanium, chromium, alumina and ammonium halide. The above ingredients are taken in the following ratio (in % by weight):
______________________________________ titanium 5 to 30 chromium 10 to 50 alumina 15 to 45 ammonium halide 10 ______________________________________
When employing the composition described above, the diffusion coating on the surface of the ferrous metals is formed in the course of heating the workpieces in an atmosphere of inert gas (argon). At a temperature of 900.degree. to 1100.degree. C. there takes place diffusion saturation of the surface layer of carbonaceous steel with titanium and chromium. Titanium forms carbides with the carbon of the substrate. It is these carbides which substantially provide the corrosion-resisting properties of the coating.
An obvious advantage of the composition described above is that a coating made therewith does not suffer from selective corrosion. In addition, free titanium and chromium are more active in diffusion saturation and their concentration in the protecting layer is much higher than that found in the priod art described earlier. However, this composition is not always efficient either. Particularly, coatings obtained with the aid of this composition are not sufficiently resistant in highly concentrated salt solutions and in chloride containing media, which is caused by the quality of the ingredients and their ratio in the composition. Thus, a comparatively low content of titanium in the mixture (up to 30% by weight) does not permit a continuous layer of carbides and a high concentration of titanium in the surface layer to be obtained. Any considerable increase in the content of titanium in the composition produce no better results, because of a large amount of chromium (weight ratio between chromium and titanium is 2:10) which binds the substrate carbon, thereby inhibiting diffusion and formation of titanium carbides. Chromium carbides contained in a large amount in the diffusion coating makes the coating highly fragile and causes microcracks therein. This is extremely unadvisable when protecting apparatus components operating in corrosive liquid media, such as pump impellers, valves, and screens. Over a period of time, insufficient density and continuity of the diffusion layer result in a pitting corrosion and destruction of the coating. This process proceeds much faster at temperatures of corrosive media from 70.degree. to 95.degree. C. It is to be noted that the temperature range often coincides with the parameters of the production processes for producing such products as magnesium chloride, sodium sulphate, Solvay soda, and barium chloride.